1. Field of the Invention
The present invention relates to an image forming device such as a printer, a copier, and a Fax, and more particularly, to an automatic belt tension apparatus of an image forming device that can automatically apply a predetermined tension to a transfer belt or a photosensitive belt only when a belt unit such as a transfer unit or an intermediate transfer unit comprising the transfer belt, or a photosensitive body comprising the photosensitive belt, is operated, and a method thereof.
2. Description of the Related Art
In general, a typical electrophotograph color image forming device is provided with: a plurality of image forming units; a plurality of photosensitive bodies, each on which a toner image of color different from one another is formed by the corresponding respective image forming units; a transfer unit having a transfer belt or a transfer roller sequentially transferring the toner images formed on the photosensitive bodies to a recording medium such as a sheet of paper; and a fusing unit fixing the toner images transferred onto the recording medium with a heat and a pressure.
Another color image forming device comprises an intermediate transfer unit having an intermediate transfer element such as a transfer belt disposed between the photosensitive bodies and the transfer unit. In this case, the toner images formed on the photosensitive bodies are not directly transferred to the recording medium, but first formed as a first transfer image on the intermediate transfer element and then transferred to the recording medium.
These color image forming devices generally use a plurality of photosensitive drums, each forming a toner image of color different from one another thereon, as the plurality of photosensitive bodies. But some image forming devices, for example, a wet type image forming device, use one photosensitive belt instead of the plurality of photosensitive drums. In this case, toner images of colors different from one another are formed on the photosensitive belt by a plurality of developing parts forming the image forming units, transferred onto the recording medium via the transfer roller of the transfer unit, and then fixed on the recording medium by the fusing unit.
Also, in these color image forming devices, a tension apparatus is used to maintain the transfer belt installed in the transfer unit or the intermediate transfer unit, or the photosensitive belt used instead of the photosensitive drums in a tensioned state of coming in contact with the photosensitive drums or the transfer roller of the transfer unit in a predetermined pressure during the transfer operation or the image forming operation.
FIGS. 1 and 2 show an intermediate transfer unit 10 of a general color image forming device using a transfer belt as an intermediate transfer element.
The intermediate transfer unit 10 is provided with: an intermediate transfer belt 9 changing toner images formed on photosensitive bodies 11Y, 11M, 11C, 11K by image forming units (not shown) into a first transfer image; first transfer rollers 15Y, 15M, 15C, 15K pressing the photosensitive bodies 11Y, 11M, 11C, 11K with a predetermined pressure with the intermediate transfer belt 9 interposed therebetween; and a transfer belt tension apparatus 20 applying a predetermined tension to the intermediate transfer belt 9 to allow the intermediate transfer belt 9 and the photosensitive bodies 11Y, 11M, 11C, 11K to be in contact with each other with a predetermined pressure.
The intermediate transfer belt 9 is rotatably installed on a driving roller 7 supported on a frame 1 by: a driving shaft 3 having a driving gear 5; supporting and backup rollers 13 and 15, which are respectively supported on the frame 1 by corresponding shafts 13a and 15a; and a second transfer backup roller 8 supported on the frame 1 by corresponding shaft 8a. 
At a lower part of the second transfer backup roller 8, a transfer roller 17 of a second transfer unit (not shown), which transfers the first transfer image formed on the intermediate transfer belt 9 onto a sheet of paper, comes in contact with the intermediate transfer belt 9 with a predetermined pressure.
The transfer belt tension apparatus 20 is provided with a swing shaft 25 having both ends supported at the frame 1, a tension roller 21 coming in contact with an inner surface of the intermediate transfer belt 9, a swing arm 23 connecting the tension roller 21 to the swing shaft 25, and a tension spring 27 imparting an elastic rotating force to the swing shaft 25 in a direction of enabling the tension roller 21, connected to the swing shaft 25 through the swing arm 23, to be in contact with the inner surface of the intermediate transfer belt 9.
The tension spring 27 has one end 27a supported in a first fixing groove 24a positioned at the swing arm 23 and an other end 27b supported in a second fixing groove 29a of a spring boss 29 positioned at the frame 1.
Accordingly, the tension spring 27 urges the tension roller 21 to always push the intermediate transfer belt 9 in a direction of arrow A of FIG. 1, and thereby the intermediate transfer belt 9 is maintained in a tensioned state to contact the photosensitive bodies 11Y, 11M, 11C, 11K with the predetermined pressure.
But, the conventional intermediate transfer unit 10 constructed as above when used for a long period of time, may present a problem. Since the tension roller 21 is continuously applying the tension to the intermediate transfer belt 9, the length of the intermediate transfer belt 9 grows longer, thereby inducing speed deviation during the transfer operation.
Also, with the conventional intermediate transfer unit 10, when left as it is without being used for a long period of time, the intermediate transfer belt 9 may generate traces at portions thereof coming in contact with the driving roller 7, the supporting and backup rollers 13 and 15, and the second transfer backup roller 8, thereby degrading a quality of the image formed during the transfer operation.
These problems may occur in the transfer unit using the transfer belt and the photosensitive body composed of the photosensitive belt, as well as the conventional intermediate transfer unit 10.